Born March 24, 1974, in Washington, D.C., Hannigan became a TV commercial veteran following her family's move to Atlanta. From the age of four, Hannigan appeared in a steady stream of commercials, including ones for Oreo and McDonald's. At the age of 11, she moved to Los Angeles to pursue a career in film and television. She made her big-screen debut in the 1988 film My Stepmother Is an Alien, as Dan Aykroyd's daughter. The following year, she won a regular spot on the short-lived sitcom Free Spirit and acted in a variety of television miniseries and sitcoms (including Roseanne and Picket Fences) until 1997, when she won the part of Willow on Buffy.
The show proved to be an unexpected success among critics and viewers alike, and paved the way for Hannigan to appear opposite Tom Everett Scott in the 1998 comedy Dead Man on Campus. American Pie followed the next year, giving Hannigan wider recognition and making her an unintentional poster child for band camps everywhere. She would return for sequels in the franchise, but Hannigan would also become just as well known for her subsuquent role on the popular sitcom How I Met Your Mother.